Method of producing a coated fiber-containing composite

ABSTRACT

A composite is produced by depositing an organic slurry of ceramic matrix-forming material on a layer of boron nitride coated fibrous material forming a tape therewith on drying, forming a layered structure of the tapes, laminating the structure, firing the laminated structure to burn out organic material and hot pressing the resulting porous structure to form a composite.

This application is related to U.S. Serial No. 132,753, filed on Dec.14, 1987, U.S. Serial No. 135,858, filed on Dec. 21, 1987, and SerialNo. (260201), filed Oct. 20, 1988, all for Singh et al., assigned to theassignee herein and incorporated herein by reference. The referencedapplications are directed to the production of filament-containingcomposites.

The present invention is directed to producing a boron nitride coatedfiber-reinforced ceramic matrix composite.

Fiber reinforcement of brittle ceramic materials offers significantopportunities for toughening of the brittle matrix. For this reasonceramic matrices are being incorporated into fiber preforms for thefabrication of ceramic matrix composites. Several techniques forincorporating the ceramic matrix into a fiber preform have been tried.These are: filament-winding through a slurry of the matrix material,chemical vapor infiltration and sol-gel infiltration techniques. Inpassing a filament winding through a slurry of the matrix, relativelysmall amounts of the matrix adhere to the filaments. Chemical vaporinfiltration and sol-gel infiltration techniques are slow. Conventionalceramic processing techniques such as slip casting and/or vacuum castingtechniques followed by hot-pressing do not provide good penetration ofthe matrix material between the reinforcing fiber preforms therebyleaving large voids in the preform. These difficulties are overcome bythe present invention.

In the present process, an organic slurry of ceramic matrix-formingmaterial is cast onto boron nitride coated fibrous material to form atape therewith, a plurality of the tapes are formed into a layeredstructure, which is then laminated, fired to burn out binder, andhot-pressed for consolidation.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the accompanying figures which form apart of the specification wherein:

FIG. 1 shows a graph (labelled 1A) illustrating the load deflectionbehavior of the present composite comprised of boron nitride coatedfilaments in a zircon matrix, and another graph (labelled lX)illustrating the load deflection behavior of a monolithic body ofzircon; and

FIG. 2 shows a graph (labelled 2A) illustrating the load-deflectionbehavior of the present composite comprised of whiskers and boronnitride coated filaments in a zircon matrix, and another graph (labelled2X) illustrating the load deflection behavior of a composite comprisedof zircon and whiskers.

Briefly stated, the present process for producing a composite comprises:

(a) providing matrix-forming ceramic particulate material and organicbinding material;

(b) forming a slurry of said matrix-forming ceramic material and organicbinding material in a liquid medium;

(c) depositing a coating of boron nitride on fibrous material leaving nosignificant portion thereof exposed;

(d) providing said boron nitride coated fibrous material substantiallyas a layer;

(d) casting said slurry onto said coated fibrous material in an amountsufficient to form a tape therewith;

(f) evaporating said liquid medium forming a tape;

(g) assembling a plurality of said tapes to produce a layered structure;

(h) laminating the layered structure to form a laminated structure;

(i) firing said laminated structure to remove said organic bindingmaterial leaving no significant deleterious residue; and

(j) hot pressing the resulting porous structure at a sufficienttemperature under a sufficient pressure for a sufficient period of timeto consolidate said structure to produce said composite having aporosity of less than about 10% by volume, said composite containing nosignificant amount of reaction product of said coated fibrous materialand said matrix, said matrix having a thermal expansion coefficientwhich ranges from lower than that of said coated fibrous material toless than about 15% higher than that of said coated fibrous material, atleast about 10% by volume of said composite being comprised of saidcoated fibrous material.

As used herein "fibrous material" includes fibers, filaments, continuousfiaments, strands, bundles, whiskers, cloth, felt and any combinationthereof. The fibrous material can be amorphous, crystalline or a mixturethereof. The crystalline fibrous material can be single crystal and/orpolycrystalline.

In one embodiment the fibrous material is a carbon-containing materialwhich contains carbon in an amount of at least about 1% by weight,frequently at least about 5% by weight, of the fibrous material.

In another embodiment, the fibrous material is selected from the groupconsisting of aluminum oxide, mullite, elemental carbon, aSiC-containing material, a silicon nitride-containing material, siliconnitride, and any combination thereof.

As used herein, "elemental carbon" includes all forms of elementalcarbon including graphite.

Reference herein to a fibrous material of silicon carbide, includes,among others, presently available materials wherein silicon carbideenvelops a core or substrate, and which generally are produced bychemical vapor deposition of silicon carbide on a core or substrate suchas, for example, elemental carbon or tungsten.

The SiC-containing fibrous material generally contains at least about50% by weight of silicon and at least about 25% by weight of carbon,based on the weight of the fibrous material. Examples of SiC-containingmaterials are silicon carbide, Si--C--O, Si--C--O--N, Si--C--O-Metal andSi--C--O--N--Metal, where the Metal component can vary but frequently isTi or and wherein O, N and Metal are present generally in an amount ofat least about 1% by weight of the fibrous material.

The silicon nitride-containing fibrous material contains at least about50% by weight of silicon and at least about 25% by weight of nitrogenbased on the weight of said fibrous material and is selected from thegroup consisting of Si--N--O, Si--C--O--N, Si--N--O--Metal, andSi--C--O--Metal, wherein said O, C, and Metal are each present in anamount of at least about 1% by weight of said fibrous material.

There are processes known in the art which use organic precursors toproduce SiC- and silicon nitride-containing fibrous materials which mayintroduce a wide variety of elements into the fibrous material.

The fibrous material is stable at the temperature of the presentprocess. Preferably, the fibrous material has in air at ambient or roomtemperature, i.e. from about 20° C. to about 30° C., a minimum tensilestrength of about 100,000 psi and a minimum tensile modulus of about 25million psi.

In carrying out the present process, boron nitride is coated on thefibrous material to produce a coating thereon which leaves nosignificant portion of the fibrous material exposed. The boron nitridecoating should be continuous, free of any significant porosity andpreferably it is pore-free. Preferably, the boron nitride coating is ofuniform or at least significantly uniform thickness.

The boron nitride coating can be deposited on the fibrous material by anumber of known techniques under conditions which have no significantdeleterious effect on the fibrous material. Generally, the boron nitridecoating can be deposited by chemical vapor deposition by reactions suchas:

    B.sub.3 N.sub.3 H.sub.6 (g)→3BN(s)+3H.sub.2 (g)     (1)

    B.sub.3 N.sub.3 H.sub.3 Cl.sub.3 (g)→3BN(s)+3HCl(g) (2)

    BCl.sub.3 (g)+NH.sub.3 (g)→BN(s)+3HCl(g)            (3)

Generally, the chemical vapor deposition of boron nitride is carried outat temperatures ranging from about 900° C. to 1800° C. in a partialvacuum, with the particular processing conditions being known in the artor determinable empirically.

The boron nitride coating should be at least sufficiently thick to becontinuous and sufficiently thin so that the thermal expansioncoefficient of the boron nitride coated fibrous material is the same as,or not significantly different from, that of the uncoated fibrousmaterial. The boron nitride coating should leave none, or no significantportion, of the fibrous material exposed. Generally, the thickness ofthe coating ranges from about 0.3 microns to about 5 microns, andtypically it is about 0.5 microns. The particular thickness of thecoating is determinable empirically, i.e. it should be sufficient toprevent reaction, or prevent significant reaction, between the fibrousmaterial and the matrix-forming material under the particular processingconditions used or under the particular conditions of use of theresulting hot pressed composite. In the present invention, the boronnitride coating bars contact, or bars significant contact, between thefibrous material and the matrix-forming material or matrix.

In carrying out the present process, the boron nitride coated fibrousmaterial is provided substantially as a layer. The layer of coatedfibrous material can be continuous or discontinuous and it containssufficient spacing to permit production of the present composite.Specifically, there is sufficient spacing between the coated fibers,filaments, strands, bundles, or whiskers to permit penetration thereofby the ceramic particulates to produce the present composite. The extentof the spacing in the layer of coated fibrous material is determinedempirically and depends largely on the size of the ceramic particulatesand the particular composite desired.

The matrix-forming material is comprised of ceramic particulates. Theseparticulates are inorganic, crystalline, and in the present process,they are consolidated, i.e. they undergo solid state sintering, toproduce the present solid composite. The matrix-forming particulates canbe comprised of an oxide-based ceramic such as, for example, aluminumoxide. mullite or zircon. Other suitable matrix-forming materials aresilicon carbide and silicon nitride. The particulates are of a sizewhich can penetrate the spaces in the layer of fibrous materialsufficiently to produce the present composite. Generally, the ceramicparticles have a specific surface area ranging from about 0.2 to about10 meters² per gram, and frequently, ranging from about 2 to about 4meters² per gram.

In the present invention, the matrix-forming material, or matrix in thecomposite, has a thermal expansion coefficient ranging from lower thanthat of the coated fibrous material to less than about 15% higher thanthat of the coated fibrous material. Depending on such factors as fiberor filament size, alignment of the fibers or filaments and theparticular processing conditions, a matrix-forming material with athermal expansion coefficient about 15% or more higher than that of thecoated fibrous material may result in a matrix with significantlydeleterious cracks which would render the composite substantially lessuseful. Preferably, for optimum mechanical properties of the composite,the matrix-forming material, or matrix, has a thermal expansioncoefficient ranging from less than to about the same as that of thecoated fibrous material.

In the present process the components of the composite are solid. Also,there is no significant amount of reaction product formed, or noreaction product detectable by scanning electron microscopy, between theceramic matrix and any other component of the present composite.

In one embodiment of the present process, a slurry of the ceramicparticulates and organic binding material is formed.

In another embodiment of the present process, a slurry of the ceramicparticulates, the present fibrous material in the form of whiskers,preferably crystalline inorganic whiskers, and organic binding materialis formed.

The whiskers in the slurry may or may not be coated with boron nitridedepending largely on the particular whiskers used and the particularcomposite desired. Generally, boron nitride coating on the whiskersimproves toughness of the ceramic matrix phase. Generally, the whiskersor coated whiskers are less than about 50 microns in length and lessthan about 10 microns in diameter. Generally, the whiskers or coatedwhiskers in the slurry range up to about 50%, or from about 1% to about30%, or from about 20% to about 30%, by volume of the matrix-formingmaterial. The whiskers in the slurry may or may not penetrate the spacesin the layer of fibrous material depending largely on the size of thewhiskers. Preferably, the whiskers introduced by the slurry are siliconcarbide or silicon nitride.

The organic binding material used in the present process bonds theceramic particulates and fibrous material together and enables formationof tape of desired thickness and solids content. By solids content, itis meant herein the content of matrix-forming material and fibrousmaterial including boron nitride coating. The organic binding material,i.e. that component of the tape other than its solids content, thermallydecomposes at an elevated temperature ranging to below about 800° C.,generally from about 50° C. to below about 800° C., and preferably fromabout 100° C., to about 500° C., to gaseous product of decompositionwhich vaporizes away leaving no significant deleterious residue.

The organic binding material is a thermoplastic material with acomposition which can vary widely and which is well known in the art orcan be determined empirically. Besides an organic polymeric binder itcan include an organic plasticizer therefor to impart flexibility. Theamount of plasticizer can vary widely depending largely on theparticular binder used and the flexibility desired, but typically, itranges up to about 50% by weight of the total organic content.Preferably the organic binding material is soluble in a volatilesolvent.

Representative of useful organic binders are polyvinyl acetates,polyamides, polyvinyl acrylates, polymethacrylates, polyvinyl alcohols,polyvinyl butyrals, and polystyrenes. The useful molecular weight of thebinder is known in the art or can be determined empirically. Ordinarily,the organic binder has an average molecular weight at least sufficientto make it retain its shape at room temperature and generally such anaverage molecular weight ranges from about 20,000 to about 200,000,frequently from about 30,000 to about 100,000.

Representative of useful plasticizers are dioctyl phthalate, dibutylphthalate, diisodecyl glutarate, polyethylene glycol and glyceroltrioleate.

In carrying out the present process, the matrix-forming particles andorganic binding material along with any whiskers are admixed with aliquid medium to form a suspension or slurry which preferably is uniformor at least substantially uniform. A number of conventional techniquescan be used to form the slurry. Generally, the components are milled inan organic solvent in which the organic material is soluble or at leastpartially soluble to produce a castable suspension or slurry, i.e. aslurry suitable for depositing on the layer of coated fibrous materialto form a tape therewith. Examples of suitable solvents are methyl ethylketone, toluene and alcohol.

The tape can be cast by a number of conventional techniques. Preferably,the layer of coated fibrous material is deposited on a carrier fromwhich the resulting tape can be easily released such as Teflon. Theslurry can be deposited on the layer of coated fibrous material to forma tape therewith of desired thickness and solids content which isdetermined empirically. Frequently, the slurry is cast on the layer ofcoated fibrous material by doctor blading. The cast tape is dried toevaporate the solvent therefrom to produce the present tape which isthen removed from the carrier.

The particular amount of organic binding material used in forming theslurry is determined empirically and depends largely on the amount anddistribution of solids desired in the resulting tape. Generally, theorganic binding material ranges from about 25% by volume to about 50% byvolume of the solids content of the tape.

The present tape or sheet can be as long and as wide as desired, andgenerally it is of uniform or substantially uniform thickness. Itsthickness depends largely on the volume fraction of fibrous materialwhich must be accommodated and the particular composite desired and isdeterminable empirically. The tape should be at least sufficiently thickto contain an amount of matrix-forming ceramic particulates and fibrousmaterial to produce the desired composite. Generally, with increasingvolume fractions of fibrous material, correspondingly smaller amounts ofmatrix-forming material would be required. Generally, the tape has athickness ranging from about 25 microns (0.001 inch) to about 1300microns (0.052 inch), frequently ranging from about 125 microns (0.005inch) to about 1000 microns (0.040 inch), and more frequently rangingfrom about 250 microns (0.01 inch) to about 500 microns (0.02 inch).

In one embodiment of the present invention, the fibrous material iscomprised of boron nitride coated filaments preferably with a diameterof at least about 50 microns. Preferably, the diameter of the coatedfilament ranges from about 50 microns to about 250 microns, frequentlyfrom about 70 microns to about 200 microns, or from about 100 microns toabout 150 microns. The filament is continuous and can be as long asdesired. It has a minimum length of at least about 10 times itsdiameter, and generally, it is longer than about 1000 microns, or it islonger than about 2000 microns. The minimum diameter of the coatedfilament depends largely on the minimum spacing required between thecoated filaments through which the matrix-forming particles mustpenetrate and is determined empirically. For a given volume fraction ofcoated filament, as the diameter of the coated filament decreases, thetotal amount of space between coated filaments decreases making it moredifficult for the ceramic particles to penetrate the space.

Preferably, a preform comprised of a layer of continuous boron nitridecoated filaments which are spaced from each other and which areparallel, or at least substantially parallel, to each other is used. Theminimum space between the coated filaments is at least sufficient toenable the matrix-forming material to penetrate therebetween, andgenerally, it is at least about 50 microns, and frequently at leastabout 100 microns. Generally, the spacing between coated filaments in asingle layer is substantially equivalent, or if desired, it can vary.Filament loading in the composite can be varied by changing the spacingbetween the coated filaments and/or tape thickness. In a preferredembodiment the filaments are comprised of silicon carbide or elementalcarbon and the slurry contains whiskers of silicon carbide. The presentinvention enables the production of a composite with a high volumefraction of uniaxially aligned spaced continuous coated filaments.

The preform of coated filaments can be produced by a number ofconventional techniques. For example, the coated filaments can beuniaxially aligned and spaced by placing them in a suitable deviceprovided with grooves and the desired spacing. The layer of coatedfilaments can be lifted off the device with adhesive tape placed acrossboth ends of the filaments. The slurry can then be deposited on thelayer of coated filaments to produce a tape therewith. If desired, thetaped end portions of the filaments can eventually be cut away from thelaminated structure.

In carrying out the present process, a plurality of the tapes isassembled into a layered structure. The number of tapes used can varywidely depending largely on the particular composite desired.Preferably, the tapes in the layered structure are at leastsubstantially coextensive with each other, i.e. substantially asandwich-type structure.

In one embodiment, before assembly of the layered structure, a solutionof the present organic binder in organic solvent is deposited, generallysprayed, on the faces of the tapes to be contacted with each other,dried to evaporate the solvent and leave a sticky film of organic binderto enhance adhesion. The concentration of organic binder in solution canvary widely and generally ranges from about 1% by weight to about 10% byweight of the solution. The solution is sprayed on the face of the tapefor a period of time, determinable empirically, so that on evaporationof the solvent sufficient sticky binder remains to significantly enhanceadhesion or facilitate bonding of the tapes. Preferably, drying iscarried out in air at ambient temperature in less than a minute, andtypically, in a few seconds. The deposited binder can be a continuous ora discontinuous coating, and typically, 0.2 milligrams of sticky binderper square centimeter of surface is adequate.

The layered structure is then laminated under a pressure and temperaturedetermined empirically depending largely on the particular compositionof the organic binding material to form a laminated structure.Lamination can be carried out in a conventional manner. Laminatingtemperature should be below the temperature at which there isdecomposition, or significant decomposition, of organic binding materialand generally, an elevated temperature below 150° C. is useful and thereis no significant advantage in using higher temperatures. Typically, thelamination temperature ranges from about 35° C. to about 95° C. and thepressure ranges from about 500 psi to about 3000 psi. Generally,lamination time ranges from about 1/2 to about 5 minutes. Also,generally, lamination is carried out in air.

If desired, the laminated structure can be cut to desired dimensions bysuitable means such as a diamond saw.

The laminated structure is heated to thermally decompose the organicbinding material therein producing a porous structure comprised of theboron nitride coated fibrous material, any uncoated whiskers, andceramic matrix-forming material. The rate of heating depends largely onthe thickness of the sample and on furnace characteristics. At a firingtemperature ranging up to about 500° C., a slower heating rate isdesirable because of the larger amounts of gas generated at thesetemperatures by the decomposition of the organic binding material.Typically, the heating rate for a sample of less than about 6millimeters (6000 microns) in thickness can range from about 15° C. perhour to about 30° C. per hour. At a temperature of less than about 800°C., thermal decomposition is completed leaving no significantdeleterious residue.

Thermal decomposition can be carried out in any atmosphere, preferablyat about or below atmospheric pressure, which has no significantdeleterious effect on the sample such as, for example, argon.Preferably, thermal decomposition is carried out in a partial vacuum toaid in removal of gases.

The resulting porous structure is hot pressed at a sufficienttemperature under a sufficient pressure for a sufficient period of timeto consolidate the structure to produce the present composite. Theparticular pressure, temperature and time are determinable empiricallyand are interdependent. Hot pressing temperature can vary dependinglargely on the characteristics of the matrix-forming material, theapplied pressure and hot pressing time. Generally under higher appliedpressures and longer times, lower hot pressing temperatures can be used.Likewise, under lower applied pressures and shorter times, higher hotpressing temperatures would be used. Generally, the hot pressingtemperature is at least about 1400° C., generally ranging from about1400° C. to about 1700° C., frequently from about 1500° C. to about1650° C., and more frequently from about 1550° C. to about 1600° C.Generally, temperatures below about 1400° C. are likely to produce acomposite having a porosity greater than about 5% whereas temperaturesabove about 1700° C. may coarsen the grains in the product and noteffect density.

Generally, hot pressing pressure ranges from higher than about 100 psito a maximum pressure which is limited by the creep of the sample, i.e.there should be no significant deformation by creep of the sample.Frequently, hot pressing pressure ranges from about 1OOO psi or about2000 psi to about 8000 psi. It is advantageous to use a pressure closeto the maximum available because the application of such high pressuremakes it possible to keep the pressing temperature low enough to controlgrain growth. Generally, hot pressing is carried out in a period of timeranging up to about 30 minutes and longer periods of time usually do notprovide any significant advantage.

Hot pressing is carried out in a non-oxidizing atmosphere. Moreparticularly, it is carried out in a protective atmosphere in which thesample is substantially inert, i.e. an atmosphere which has nosignificant deleterious effect thereon. Representative of the hotpressing atmospheres is nitrogen, argon, helium or a vacuum. The hotpressing atmosphere generally can range from a substantial vacuum toabout atmospheric pressure.

In the present process, there is no loss, or no significant loss, of thecomponents forming the present composite, i.e. boron nitride coatedfibrous material, any uncoated whiskers, and matrix-forming material.

In one embodiment, the present composite is comprised of ceramic matrixand boron nitride coated fibrous material. In another embodiment, thecomposite is comprised of ceramic matrix, boron nitride coated fibrousmaterial and whiskers which are not coated with boron nitride. Theceramic matrix is continuous and interconnecting. It is distributed inthe coated fibrous material, as well as any whiskers, and generally itis space filling or substantially completely space filling. Generally,the matrix is in direct contact with more than 70% of the surface areaof the boron nitride coated fibrous material and any uncoated whiskers.Frequently, the ceramic matrix coats or envelops each coated fiber,filament, strand, bundle or whisker of the boron nitride coated fibrousmaterial and any uncoated whisker sufficiently to be in direct contactwith more than 80%, preferably more than 90%, more preferably more than99%, of the surface area of the boron nitride coated fibrous materialand any uncoated whisker in the composite.

The ceramic matrix is present in the composite in an amount of at leastabout 30% by volume of the composite. The matrix is comprised of a solidstate sintered polycrystalline phase. Preferably, the ceramic matrixphase has an average grain size of less than about 100 microns, or lessthan about 50 microns, or less than about 20 microns, and mostpreferably less than about 10 microns.

The boron nitride coated fibrous material comprises at least about 10%by volume of the composite. Generally, the boron nitride coated fibrousmaterial, or boron nitride coated fibrous material and any uncoatedwhiskers, ranges from about 10% or greater than about 10% by volume toabout 70% by volume, frequently from about 20% by volume to about 60% byvolume, or from about 30% by volume to about 50% by volume, of thecomposite.

The boron nitride coating on the fibrous material in the composite isdetectable by scanning electron microscopy and generally ranges inthickness from about 0.5 microns to about 1.5 microns. The particularamount of boron nitride in the composite provided by the boron nitridecoating depends largely on the amount of coated fibrous materialpresent, the thickness of the boron nitride coating and the diameter ofthe fiber, filament, or whisker. Therefore, the volume fraction of boronnitride provided by the coating is the balance of the volume fraction ofall other components of the composite. Frequently, however, the boronnitride coating on the fibrous material in the composite generallyranges from less than about 1% by volume to about 20% by volume, or fromabout 1% by volume to about 10% by volume, or from about 1% by volume toabout 5% by volume, of the total volume of boron nitride coated fibrousmaterial. The boron nitride coating can be amorphous, crystalline, or acombination thereof.

The boron nitride coating generally optimizes interfacial shear stressbetween the fibrous material and ceramic matrix resulting in a compositewith a toughness significantly higher than that of a composite whereinthe fibrous material is uncoated.

In one embodiment, the present composite is comprised of a plurality oflayers of boron nitride coated fibrous material in the ceramic matrixwherein the coated fibrous layers are substantially parallel to eachother and separated from each other by ceramic matrix. The ceramicmatrix is distributed in each layer of coated fibrous material generallysignificantly or substantially uniformly. In addition to the layers ofboron nitride coated fibrous material, the ceramic matrix may containcoated or uncoated whiskers, which had been incorporated by the slurry,which may range up to about 50%, frequently from about 1% to about 30%,or from about 20% to about 30%, by volume of the ceramic matrix.Generally, the whiskers are contained in the ceramic matrix betweenlayers of boron nitride coated fibrous material, i.e. at least betweentwo layers of coated fibrous material. The whiskers may also be detectedwithin the boron nitride coated fibrous layer depending largely on shapeand size of the whiskers, spacing contained in the coated fibrous layer.Any difference in composition between the coated fibrous layer andwhiskers and particle size of the ceramic.

In another embodiment, the composite contains a plurality of layers ofboron nitride coated filaments, there is no contact between the layersand they are separated by ceramic matrix. In each layer, more than 99%by volume of the coated filaments, and preferably all or substantiallyall of the coated filaments, are spaced from each other and parallel orat least substantially parallel, to each other. More than 99% by volumeor substantially all of the coated filaments in each layer are aligned,or substantially aligned, in a single plane. Any misalignment of thecoated filaments should not significantly degrade the mechanicalproperties of the composite. Also, more than 99% or substantially all ofthe surface area of the coated filaments is in direct contact with theceramic matrix. Also, boron nitride coated and/or uncoated whiskers maybe present up to about 50% by volume of the ceramic matrix.

The boron nitride coating optimizes interfacial shear stress between thefilaments and matrix resulting in a composite with a toughnesssignificantly or substantially higher than that of a composite whereinthe filaments are uncoated. Specifically, if the matrix and filamentswere in direct contact, even a slight reaction therebetween wouldincrease interfacial bonding thereby requiring a higher stress to pullout the filaments making the composite less tough. If the interfacialbonding were too high, then the composite would fail in a brittlemanner. In contrast, the present boron nitride coating provides aninterfacial shear stress which is significantly lower than that producedwith uncoated filaments thereby allowing the coated filaments to pullout more easily and gives the composite more toughness. The coatedfilaments prevent brittle fracture of the composite at room temperature.By brittle fracture of a composite it is meant herein that the entirecomposite cracks apart at the plane of fracture. In contrast to abrittle fracture, this embodiment of the composite exhibits filamentpull-out on fracture at room temperature. Specifically, as thiscomposite cracks open, generally at least about 10% by volume,frequently at least about 30% or 50% by volume, of the coated filaments,and preferably all of the coated filaments, pull out and do not break atthe plane of fracture at room temperature.

One particular advantage of this invention is that the present compositecan be produced directly in a wide range of sizes. For example, it canbe as long or as thick as desired.

The present composite has a porosity of less than about 10%, preferablyless than about 5%, more preferably less than about 1%, by volume of thecomposite. Most preferably, the composite is void- or pore-free, or hasno significant porosity, or has no porosity detectable by scanningelectron microscopy. Generally, any voids or pores in the composites areless than about 70 microns, preferably less than about 50 microns orless than about 10 microns, and generally they are distributed in thecomposite.

The present composite has a wide range of applications depending largelyon its particular composition. For example, it is useful as a wearresistant part, acoustical part or high-temperature structuralcomponent.

The invention is further illustrated by the following examples where,unless otherwise stated, the procedure was as follows:

Commercially available continuous filaments of silicon carbide producedby a chemical vapor deposition process and sold under the trademark AVCOSCS-6 were used. These filaments had a 35 micron carbon core on whichsilicon carbide was deposited to an overall diameter of about 145microns. The outside surface of the filaments consisted of two layers ofpyrolytic carbon and carbon-silicon, with overall thickness of about 3microns. In air at room temperature these filaments have a tensilestrength of about 500 thousand psi and a tensile modulus of about 60million psi. These filaments have an average thermal expansioncoefficient of less than about 5.0×10⁻⁶ in/in-° C.

The filaments were cut to a length of about 2 inches and were coatedwith boron nitride by the following low pressure chemical vapordeposition process utilizing the reaction B₃ N₃ H₃ Cl₃ →3BN +3HCl.Specifically, the filaments were placed on a molybdenum screen which wasthen positioned at about the midpoint of the hot zone of apyrex/quartz/pyrex furnace tube. A 1.00 gram sample of commercialtrichloroborazine (B₃ N₃ H₃ Cl₃) was transferred in an argon-filledglove box to a pyrex end-section which contained a thermocouple vacuumgauge, a cold trap and a vacuum stopcock. The closed pyrex end-sectionwas then taken out of the glove box and attached to an end of thefurnace tube and to a vacuum system. The end-section containing thetrichloroborazine was then cooled using liquid nitrogen and the furnacetube was opened to the vacuum system via the stopcock of the pyrexend-section. After the system reached a pressure lower than 0.020 torr,the furnace was heated to about 1050° C. When the pressure had againdropped below 0.020 torr and the furnace temperature had stabilized, theend-section containing the trichloroborazine was warmed by an oil bathmaintained at 60° C., whereupon the solid began to vaporize, depositingBN and liberating gaseous HCl in the hot zone of the furnace tube andproducing an increase in pressure. The pressure was observed to reach ashigh as about 200 torr before stabilizing at about 50 torr. After twohours, the pressure was found to have decreased to about 0.020 torr,whereupon the furnace was shut down and the system allowed to cool toroom temperature before opening the tube and removing the sample.Identification of the chemically vapor deposited layer as BN wasaccomplished by means of electrical resistance measurement and aquantitative ESCA analysis of a film deposited in substantially the samemanner on a SiC disk surface. This film was amorphous to x-rays in theas-deposited condition and appeared fully dense and smooth at highmagnification in the SEM. Scanning electron microscopy observation ofthe ends of coated and broken filaments revealed that the coating wascontinuous and smooth and about 1.5 microns thick on the filament andleft no significant portion of the filament exposed.

The boron nitride coated filaments were uniaxially aligned by placingthem in a device for aligning filaments and maintaining the requiredspacing between them. This device was made from a copper foil laminatedon a printed circuit board which was etched by the photolithographictechnique in such a way as to produce parallel grooves about 0.06 inchdiameter, 0.0004 inch deep, and 0.0008 inch apart (center-to-center).The coated filaments were placed on this device and a simple scoop ofthe filaments using a straight edge led to filling of each of thegrooves with a filament. This resulted in a single layer of uniformlyspaced coated filaments which was lifted off the board by puttingadhesive tapes across each end portion of the filament layer. Theadhesive tapes were sufficient to maintain the alignment and spacingbetween the coated filaments in the layer. Several such pre-formedlayers of coated filaments were produced in which the coated filamentswere substantially parallel and spaced about 100 microns from eachother.

The zircon powder had an average size of about 0.5 microns.

By ambient temperature herein it is meant room temperature, i.e. fromabout 20° C. to about 30° C.

The organic binding material was comprised of commercially availableorganic binder comprised of polyvinylbutyral (average molecular weightof about 32,000) and commercially available liquid plasticizer comprisedof polyunsaturated hydroxylated low-molecular weight organic polymers.Specifically, in Example 1, the organic binding material was comprisedof 8.75 grams of polyvinylbutyral and 7.9 grams of liquid plasticizer,and in Example 2 it was comprised of 9.4 grams of polyvinylbutyral and11.0 grams of liquid plasticizer.

Hot pressing was carried out in a 2 inch inner diameter 2 inch innerlength cylindrical die in an atmosphere of flowing nitrogen which was atabout atmospheric pressure.

Standard techniques were used to characterize the hot pressed compositefor density, microstructure and mechanical properties.

EXAMPLE 1

23.8 grams of the organic binding material were dissolved at ambienttemperature in 76.2 grams of a mixture of 53.9 grams of toluene, 18.3grams of methyl isobutyl ketone, and 4.0 grams of ethyl alcohol. The 70grams of resulting solution was admixed with 200 grams of zircon powderalong with 38 grams of toluene and 0.25 grams of a commerciallyavailable organic silicone oil (which can be considered a part of theorganic binder) in a ball mill for about 10 hours at room temperature toform a slurry. The slurry was then desired in a vacuum. Each pre-formedlayer of filaments was deposited on a Mylar sheet, the slurry wasdeposited on the filaments using a doctor blade, the cast tape was thendried in air at room temperature and atmospheric pressure to remove thesolvent, and the resulting tape was stripped from the Mylar sheet.

The tape was about 6 inches wide and had a substantially uniformthickness of about 0.011 inch. micron powder was distributed thereinsubstantially uniformly.

The tape was cut to the length and width of the aligned layer offilaments. A number of such tapes were produced.

A layered sandwich-type structure was formed comprised of six layers oftape. Before assembly, to enhance adherence, the faces of the tapeswhich were to be contacted with each other were sprayed with an organicsolution of binder, dried for a few seconds in air at room temperatureleaving a coating of sticky organic binder. Specifically, a solutioncomprised of 3 weight % of commercially available polyvinylbutyral(average molecular weight of about 32,000), 39 weight % toluene, 9.5weight % acetone, 39 weight % xylene and 9.5 weight % ethanol was used.The solution was sprayed on the faces of the tapes for a sufficient timeso that upon evaporation of the solvent there remained about 0.2milligrams of sticky organic binder per square centimeter of surface.

The resulting layered structure was laminated in air in a laminatingpress at about 93° C. under a pressure of about 1000 psi for about 1minute. At lamination temperature and pressure, the tapes were plasticresulting in filling of the void space between and around the filaments.

The laminated structure was sliced perpendicular to the filament axisinto bar-shaped samples (1.25 inch×0.3 inch×0.15 inch) using a diamondsaw. Examination of a cross-section showed uniform spacing between thefilaments as well as between the layers of filaments.

The samples were placed in a vacuum oven for removing the organicbinding material wherein the vacuum was typically about 20 millitorr.The burnout cycle was comprised of heating the furnace at a rate of 30°C. per hour to 500° C., a five hour hold at 500° C. and a cooldown toroom temperature at a rate of 200° C. per hour. This led to completeremoval of the organic binding matter from the laminated structure whichresulted in a porous structure comprised of zircon powder and filaments.

Each of the porous bar-shaped structures was placed in a graphite dieand hot-pressed at about 1580° C. Each sample was heated at a rate ofapproximately 100° C. per minute to the maximum hot pressing temperatureunder a pressure of 3500 psi applied for consolidation. Theconsolidation was monitored by plunger displacement and completedensification occurred within 30 minutes after the onset ofdensification. After hot-pressing, the sample was furnace cooled to roomtemperature and removed from the die.

The hot pressed samples, i.e. composites, were characterized and areillustrated in Table I.

The cross section (perpendicular to filament axis) of one of thecomposites (Example 1C) was examined. It showed uniform spacing betweenthe filaments as well as uniform spacing between layers of filamentswhich the present composite can have. It also showed that each layer offilaments was maintained in a substantially single plane. In addition,it showed a fully dense zircon matrix surrounding each individualfilament and in direct contact therewith. The density of this compositewas 4.27 g/cc, in line with fully dense zircon matrix materialcontaining about 25 volume % filaments. No porosity was detected in thecomposite by microscopy. Zircon has an average thermal expansioncoefficient of about 4.8×10⁻⁶ in/in-° C. which is close to that of thesilicon carbide filaments.

Some of the composites were broken at room temperature in athree-point-bend configuration to determine fracture strength andload-elongation characteristics. All of the broken composites exhibitedfilament pullout, i.e. more than 10% by volume of the filaments pulledout and did not break at the plane of fracture. The results for each ofthe three composites are given in Table I as Examples 1A-1C.

FIG. 1 shows a load deflection curve for the composite of Example 1A. Itcan be seen that this composite showed toughened ceramic-like behavior.The load-deflection curve shows that at the onset of matrix cracking,the load carrying capability of the composite was maintained for a whilereaching an ultimate strength of 101,000 psi (700 MPa) beyond which thecomposite showed substantial but not complete failure.

For comparison, six tapes of zircon powder alone and organic binder wereproduced, formed into a layered structure, laminated, heated to removeorganic binder, and hot pressed in substantially the same manner as thesample of Example 1A to produce a body (Example 1X in Table I) ofsubstantially the same size and density which was broken insubstantially the same manner. Its load deflection curve is also shownin FIG. 1. It fractured in a brittle manner at 35,400 psi (244 MPa).

EXAMPLE 2

This example was carried out in substantially the same manner as Example1 except as noted herein and in Table I.

Zircon powder and silicon carbide whiskers were used instead of zirconalone in forming the slurry. Specifically, 200 grams of zircon powderand 35 grams of silicon carbide whiskers were mixed by hand. To this 3grams of organic binder, 83 grams of toluene, and 38 grams of methylisobutyl ketone were added. This mixture was mixed for 20 minutes usingzirconia grinding media on a ball mill. Additional 17.4 grams of binderand 0.2 gram of silicone oil were added to the slurry and the contentswere mixed for one hour on a ball mill. The slurry was desired and castover the deposited filaments on a mylar sheet using a doctor blade.

The dried tape had a thickness of about 0.011 inch.

A number of composites were produced and some were broken. All of thebroken composites showed toughened ceramic-like behavior and filamentpullout, i.e. more than 10% by volume of the filaments pulled out anddid not break in the plane of the fracture. The results for each of thethree composites are given as Examples 2A-2C in Table I. Zircon and SiCwhiskers have an average thermal expansion coefficient of about 4.8×10⁻⁶in/in-° C. which is close to that of the filaments.

FIG. 2 shows a load deflection curve for the composite of Example 2A. Itcan be seen that this composite showed toughened ceramic-like behavior.The load-deflection curve shows the onset of matrix cracking followed bya rise in the load carrying capability of the composite. An ultimatestrength of 429 MPa (62,219 psi) was reached beyond which the compositeshowed substantial but not complete failure.

For comparison, six tapes of organic binder and a mixture of zirconpowder and SiC whiskers in an amount of about 15% by weight of themixture were produced, formed into a layered structure, laminated,heated to remove organic binder, and hot pressed in substantially thesame manner as the sample of Example 2A to produce a body (Example 2X inTable I) of substantially the same size and density which was broken insubstantially the same manner. Its load deflection curve is also shownin FIG. 2. It fractured in a brittle manner at 320 MPa (46,410 psi).

                                      TABLE 1                                     __________________________________________________________________________                  BN-Coated                                                                     Filament                                                                  Tape                                                                              Containing                                                                          Composite Characteristics                                    Solids Thick-                                                                            Tapes in                                                                            Hot Pressing      Avg.                                                                              Fracture                                                                           Fracture                          in     ness                                                                              Layered                                                                             Temperature                                                                          Density                                                                            Filaments                                                                           Grain                                                                             Strength                                                                           Strain                         Ex.                                                                              Slurry (inch)                                                                            Structure                                                                           (°C.)                                                                         g/cc Vol. %                                                                              Size                                                                              (MPa)                                                                              (%)                            __________________________________________________________________________    1A zircon 0.011                                                                             6     1560   4.31 25    <5 μm                                                                          700  1.4                            1B zircon 0.011                                                                             6     1560   4.24 25    <5 μm                                                                          650  1.0                            1C zircon 0.011                                                                             6     1560   4.27 25    <5 μm                                                                          700  1.2                            1X zircon 0.012                                                                             none  1560   4.4  0     <5 μm                                                                          244  0.09                           2A zircon +                                                                             0.011                                                                             6     1600   3.76 25    <5 μm                                                                          429  1.4                               SiC whiskers                                                               2B zircon +                                                                             0.011                                                                             6     1600   3.80 25    <5 μm                                                                          638  1.64                              SiC whiskers                                                               2C zircon +                                                                             0.011                                                                             6     1600   3.75 25    <5 μm                                                                          657  1.50                              SiC whiskers                                                               2X zircon +                                                                             0.011                                                                             none  1600   3.88 0     <5 μm                                                                          320  0.2                               SiC whiskers                                                               __________________________________________________________________________

The composites of Examples 1A, 1B, 1C, 2A, 2B, and 2C illustrate thepresent invention. The present composites are useful as high temperaturestructural materials.

What is claimed is:
 1. A process for producing a composite comprised ofboron nitride coated fibrous material in a ceramic matrix whichcomprises the following steps:(a) providing matrix-forming ceramicparticulate material and organic binding material; (b) forming a slurryof said matrix-forming material and organic binding material in a liquidmedium; (c) depositing a coating of boron nitride on fibrous materialleaving no significant portion thereof exposed; (d) providing said boronnitride coated fibrous material substantially as a layer; (e) castingsaid slurry onto said coated fibrous material in an amount sufficient toform a tape therewith; (f) evaporating said liquid medium forming atape; (g) assembling a plurality of said tapes to produce a layeredstructure; (h) laminating the layered structure to form a laminatedstructure; (i) heating said laminated structure to remove said organicbinding material leaving no significant deleterious residue; and (j) hotpressing the resulting porous structure at a sufficient temperatureunder a sufficient pressure for a sufficient period of time toconsolidate said structure to produce said composite having a porosityof less than about 10% by volume, said composite containing nosignificant amount of reaction product of said coated fibrous materialand said matrix, said ceramic matrix having a thermal expansioncoefficient which ranges from lower than that of said coated fibrousmaterial to less than about 15% higher than that of said coated fibrousmaterial, at least about 10% by volume of said composite being comprisedof said coated fibrous material.
 2. The process according to claim 1,wherein said fibrous material contains at least about 1% by weight ofcarbon.
 3. The process . . . silicon carbide, silicon nitride, and anycombinations thereof.
 4. The process . . . Si-C-O-metal, Si-C-O-N-metal,and any combinations thereof, where . . . material.
 5. The process . . .Si-N-O-metal, Si-C-O-N-metal and any combinations thereof, wherein . . .material.
 6. The process according to claim 1, wherein said fibrousmaterial is selected from the group consisting of fibers, filaments,strands, bundles, whiskers, cloth, felt, and any combination thereof. 7.The process according to claim 1, wherein said slurry contains a memberselected from the group consisting of whiskers and boron nitride coatedwhiskers ranging to about 50% by volume of said matrix-forming ceramicmaterial.
 8. The process according to claim 1, wherein saidmatrix-forming material is comprised of an oxide-based crystallineceramic.
 9. The process according to claim 1, wherein saidmatrix-forming ceramic particulate material is selected from the groupconsisting of aluminum oxide, mullite, silicon carbide, silicon nitride,and zircon.
 10. The process according to claim 1, wherein before saidlayered structure is assembled, an organic solution of organic binder issprayed on faces of tapes to be contacted with each other and dried toleave a sticky film of organic binder in amount sufficient to at leastsignificantly enhance adhesion.
 11. The process according to claim 1,wherein said matrix has a thermal expansion coefficient ranging fromlower than that of said coated fibrous material to about the same asthat of said coated fibrous material.
 12. The process according to claim1, wherein said coated fibrous material is comprised of a plurality ofcontinuous coated filaments wherein said coated filaments are spacedfrom each other.
 13. The process according to claim 1, wherein saidboron nitride coated fibrous material is comprised of a plurality ofcontinuous coated filaments spaced from each other, and wherein saidslurry contains a member selected from the group consisting of whiskersand boron nitride coated whiskers ranging to about 50% by volume of saidmatrix-forming ceramic material.
 14. The process according to claim 1,wherein said matrix-forming ceramic particulate material is mullite. 15.The process according to claim 1, wherein said matrix-forming ceramicparticulate material is silicon carbide.
 16. The process according toclaim 1, wherein said matrix-forming ceramic particulate material issilicon nitride.
 17. The process according to claim 1, wherein saidslurry contains whiskers ranging to about 50% by volume of saidmatrix-forming ceramic material.
 18. The process according to claim 1,wherein said slurry contains silicon carbide whiskers ranging to about50% by volume of said matrix-forming ceramic material.
 19. The processaccording to claim 1, wherein said slurry contains silicon nitridewhiskers ranging to about 50% by volume of said matrix-forming ceramicmaterial.
 20. A process for producing a composite comprised of boronnitride coated fibrous material in a zircon matrix which comprises thefollowing steps:(a) providing matrix-forming zircon particulate materialand organic binding material; (b) forming a slurry of saidmatrix-forming material and organic binding material in a liquid medium;(c) depositing a coating of boron nitride on fibrous material leaving nosignificant portion thereof exposed; (d) providing said boron nitridecoated fibrous material substantially as a layer; (e) casting saidslurry onto said coated fibrous material in an amount sufficient to forma tape therewith; (f) evaporating said liquid medium forming a tape; (g)assembling a plurality of said tapes to produce a layered structure; (h)laminating the layered structure to form a laminated structure; (i)heating said laminated structure to remove said organic binding materialleaving no significant deleterious residue; and (j) hot pressing theresulting porous structure at a sufficient temperature under asufficient pressure for a sufficient period of time to consolidate saidstructure to produce said composite having a porosity of less than about10% by volume, said composite containing no significant amount ofreaction product of said coated fibrous material and said matrix, saidmatrix having a thermal expansion coefficient which ranges from lowerthan that of said coated fibrous material to less than about 15% higherthan that of said coated fibrous material, at least about 10% by volumeof said composite being comprised of said coated fibrous material. 21.The process according to claim 20, wherein said fibrous materialcontains at least about 1% by weight of carbon.
 22. The processaccording to claim 20, wherein said fibrous material is selected fromthe group consisting of aluminum oxide, mullite, elemental carbon,silicon carbide, silicon nitride, and any combinations thereof.
 23. Theprocess according to claim 20, wherein said fibrous material contains atleast about 50% by weight of silicon and at least about 25% by weight ofcarbon based on the weight of said fibrous material and is selected fromthe group consisting of Si--C--O, Si--C--O--N, Si--C--O--Metal,Si--C--O--N--Metal, and any combinations thereof, wherein said O, N andmetal are each present in an amount of at least about 1% by weight ofsaid fibrous material.
 24. The process according to claim 20, whereinsaid fibrous material contains at least about 50% by weight of siliconand at least about 25% by weight of nitrogen based on the weight of saidfibrous material and is selected from the group consisting of Si--N--O,Si--C--O--N, Si--N--O--Metal, Si--C--O--N--Metal, and any combinationsthereof, wherein said O, C and Metal are each present in an amount of atleast about 1% by weight of said fibrous material.
 25. The processaccording to claim 20, wherein said fibrous material is selected fromthe group consisting of fibrous, filaments, strands, bundles, whiskers,cloth, felt, and any combinations thereof.
 26. The process according toclaim 20, wherein said slurry contains boron nitride coated whiskersranging to about 50% by volume of said matrix-forming zircon material.27. The process according to claim 20, wherein said coated fibrousmaterial is comprised of a plurality of continuous coated filamentswherein said coated filaments are spaced from each other.
 28. Theprocess according to claim 20, wherein said boron nitride coated fibrousmaterial is comprised of a plurality of continuous coated filamentsspaced from each other, and wherein said slurry contains a memberselected from the group consisting of whiskers and boron nitride coatedwhiskers ranging to about 50% by volume of said matrix-forming zirconmaterial.
 29. The process according to claim 20, wherein said slurrycontains whiskers ranging to about 50% by volume of said matrix-formingzircon material.
 30. The process according to claim 20, wherein saidslurry contains silicon carbide whiskers ranging to about 50% by volumeof said matrix-forming zircon material.
 31. The process according toclaim 20, wherein said slurry contains silicon nitride whiskers rangingto about 50% by volume of said matrix-forming zircon material.
 32. Aprocess for producing a composite comprised of boron nitride coatedfibrous material in an aluminum oxide matrix which comprises thefollowing steps:(a) providing matrix-forming aluminum oxide particulatematerial and organic binding material; (b) forming a slurry of saidmatrix-forming material and organic binding material in a liquid medium;(c) depositing a coated of boron nitride on fibrous material leaving nosignificant portion thereof exposed; (d) providing said boron nitridecoated fibrous material substantially as a layer; (e) casting saidslurry onto said coated fibrous material in an amount sufficient to forma tape therewith; (f) evaporating said liquid medium forming a tape; (g)assembling a plurality of said tapes to produce a layered structure; (h)laminating the layered structure to form a laminated structure; (i)heating said laminated structure to remove said organic binding materialleaving no significant deleterious residue; and (j) hot pressing theresulting porous structure at a sufficient temperature under asufficient pressure for a sufficient period of time to consolidate saidstructure to produce said composite having a porosity of less than about10% by volume, said composite containing no significant amount ofreaction product of said coated fibrous material and said matrix, saidmatrix having a thermal expansion coefficient which ranges from lowerthan that of said coated fibrous material to less than about 15% higherthan that of said coated fibrous material, at least about 10% by volumeof said composite being comprised of said coated fibrous material.